<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .bb {
            position: relative;
            width: 150px;
            height: 20px;
            border: 2px solid black;
        }
        
        .bb .erz {
            position: absolute;
            width: 100%;
            height: 2px;
        }
    </style>
</head>

<body>
    <div class="bb">
        <div class="erz"></div>
    </div>
</body>

</html>